In silk screening printing a synthetic squeegee blade, typically a cast polyurethane material, is used to scrape the ink from one end of the screening to the other end to accomplish printing. The wear edges of these synthetic squeegee blades wear out after a period of use due to the friction between the squeegee blade and the screen. This results in a printing area with a non-uniform ink density. Therefore, the wear edge of the squeegee blade must be replaced. This is done by replacing the squeegee blade or sharpening the blade edge after a period of operation. Additionally, the flatness of the squeegee blade is important in maintaining the quality of the printed product, so it is necessary while sharpening the wear edge of the squeegee blade to maintain its flatness. Similar concerns as to blade flatness and blade sharpening exist in other applications of rubber, plastic or polyurethane blades.
One technique used to prolong the use of a squeegee blade is to sharpen the edges by grinding material from the squeegee blade to provide a sharp edge. The conventional device is a grinding machine in which a grinding wheel is used to grind off the worn edge from the squeegee blade to create a sharp new wear edge. There are, however, a number of disadvantages to the grinding process. The process is time consuming and difficult to control because the rotation of the grinding wheel is usually very high. If the grinding wheel is moved too slowly in relation to the material, the edge of the synthetic squeegee blade undergoing sharpening may melt from the frictional heat caused by grinding wheel. If, on the other hand, the grinding wheel is moved too quickly, it is impossible to grind off the worn out edge of the squeegee blade on a once through basis; and if the grinding wheel is not moved uniformly, the sharpened edge of the squeegee blade becomes uneven which also results in a defective product.
Illustrative of grinding machines of this type are those from Ernst W. Dorn Company, Inc., Printck Equipment Co., Thieme, American CCS Microsharp and Curtis Markus, Inc.
Attempts have been made in the prior art to overcome the deficiencies associated with the use of a grinding wheel by using a knife or cutting blade to trim a squeegee. In U.S. Pat. No. 4,334,448, in particular, a heated blade is used to cut through and provide a new wear edge to a squeegee blade. The use of a heated blade causes problems of its own, however. Specifically, the use of a heated blade raises both safety and maintenance concerns. With respect to the former, the heated cutting tool raises problems with respect to possible burns if contact with the heated cutting tool occurs. In addition, the heated blade produces fumes during the cutting process, which requires ventilation. Maintenance problems include the fact that cooling must occur before cutting tools can be changed. In addition, the requirement of heat necessarily brings electrical elements into the device which present inherent maintenance and service concerns. Finally, because of the need for a heated cutting tool, the device also requires access to a power source which creates limitations in its use. Further problems exist with the heated cutting tool because the heated blade causes deformation and melting of the squeegee blade. As a result, an uneven wear edge can form on the squeegee blade.
Other prior art has attempted to use a non-heated cutting tool to obtain a flat wear edge for the squeegee blade. However, such prior art devices have been cumbersome and inefficient. In particular, certain prior art devices have required removal of the squeegee blade from the squeegee handle before sharpening can occur. Other prior art devices lack a high degree of precision in measuring the thickness of material to be trimmed from the squeegee blade. Prior art apparatuses attempting to provide a high degree of precision for such measurement have been cumbersome to operate due to the size of such apparatuses and the number of moving parts.
Accordingly, it is an object of the present invention to provide a sharpening device which is, capable of not only sharpening the wear edge of a rubber, plastic or polyurethane blade such as a synthetic squeegee blade in a high efficient and easy to control manner but capable of maintaining the flatness of the wear edge of the blade as well.